Avoid infinite recursion when raising SIGABRT in SIGABRT handler

When VM state is corrupted enough, we can call abort() from the SIGABRT
handler. Previously, we would spam until the stack is full:

    ABRT received in SEGV handler
    SEGV received in ABRT handler
    ABRT received in SEGV handler
    ABRT received in ABRT handler
    ABRT received in ABRT handler
    ABRT received in ABRT handler
    [...]

We've since this on CI on for example: https://github.com/ruby/ruby/actions/runs/26591192708/job/78350130653

To test this situation locally, temporarily patch in a call to abort()
in rb_bug_for_fatal_signal() then use `Process.kill(:ABRT, Process.pid)`.

Fix by restoring the default signal handler before aborting.

Author: XrXr

signal.c

@@ -1020,8 +1020,20 @@ check_reserved_signal_(const char *name, size_t name_len, int signo)
 #if __has_feature(address_sanitizer) || \
     __has_feature(memory_sanitizer) || \
     defined(HAVE_VALGRIND_MEMCHECK_H)
-        ruby_posix_signal(signo, SIG_DFL);
+# define FIRST_COND true
+#else
+# define FIRST_COND false
+#endif
+
+#ifdef SIGABRT
+// Avoid infinite loop when already aborting
+# define SECOND_COND (signo == SIGABRT)
+#else
+# define SECOND_COND false
 #endif
+        if (FIRST_COND || SECOND_COND) ruby_signal(signo, SIG_DFL);
+# undef FIRST_COND
+# undef SECOND_COND
         W(name, name_len);
         W(msg1, sizeof(msg1));
         W(prev, strlen(prev));